分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Recently, much attention has been paid to second-order photonic topological insulators (SPTIs), because of their support for highly localized corner states with excellent robustness. SPTIs have been implemented in either transverse magnetic (TM) or transverse electric (TE) polarizations in two-dimensional (2D) photonic crystals (PCs), and the resultant topological corner states are polarization-dependent, which limits their application in polarization-independent optics. However, to achieve polarization-independent corner states is not easy, since they are usually in-gap and the exact location in the topological bandgap is not known in advance. Here, we report on a SPTI based on a 2D square-lattice PC made of an elliptic metamaterial, and whether the bandgap is topological or trivial depends on the choice of the unit cell. It is found that locations of topological bandgaps of TM and TE polarizations in the frequency spectrum can be independently controlled by the out-of-plane permittivity $\varepsilon_\perp$ and in-plane permittivity $\varepsilon_{\varparallel}$, respectively, and more importantly, the location of in-gap corner states can also be separately manipulated by them. From this, we achieve topological corner states for both TM and TE polarizations with the same frequency in the PC by adjusting $\varepsilon_\perp$ and $\varepsilon_\varparallel$, and their robustness against disorders and defects are numerically demonstrated. The proposed SPTI provides a potential application scenario for polarization-independent topological photonic devices.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： Strong coupling exhibits unique ability to preserve quantum sates between light and matter, which is essential for the development of quantum information technology. To explore the physical mechanism behind this phenomenon, we employ the tight-binding method for expanding the temporal coupled-mode theory, with the absorption spectrum formula of coupled system directly obtained in an analytical way. It reveals all the physical meaning of parameters defined in our theory, and shows how to tailor lineshapes of the coupled systems. Here, we set an example to manipulate the strong coupling in a hybrid structure composed of excitons in monolayer WS$_2$ and quasi-bound states in the continuum supported by the TiO$_2$ nanodisk metasurfaces. The simulated results show that a clear spectral splitting appeared in the absorption curve, which can be controlled by adjusting the asymmetric parameter of the nanodisk metasurfaces and well fitted through our theoretical predictions. Our work not only gives a more comprehensive understanding of such coupled systems, but also offers a promising strategy in controlling the strong light-matter coupling to meet diversified application requests.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： We investigate the strong coupling between the excitons and quasi-bound states in the continuum (BIC) resonance in a bulk WS$_2$ metasurface. Here we employ the bulk WS$_2$ to construct an ultrathin nanodisk metasurface, supporting the symmetry-protected magnetic dipole (MD) quasi-BIC resonance, which can self-hybridize with the excitons and lead to a strong light-matter interaction enhancement within the structure without the necessity for an external cavity. This strong coupling can be charactered by the considerable Rabi splitting of 159 meV and the clearly anti-crossing behavior appeared in the absorption spectrum. Furthermore, we analyze such light-matter coupling by constructing a Hamiltonian model including the surplus excitons, and tune the interaction from weak coupling to strong coupling regimes via the tunability radiation loss of the quasi-BIC resonance. Our results have great potential for manipulating the exciton-polaritons at room temperature, and provide a promising prospect for photonic devices that exploit strong coupling in applications.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： The emerging all-dielectric platform exhibits high-quality ($Q$) resonances governed by the physics of bound states in the continuum (BIC) that drives highly efficient nonlinear optical processes. Here we demonstrate the robust enhancement of third-(THG) and fifth-harmonic generation (FHG) from all-dielectric metasurfaces composed of four silicon nanodisks. Through the symmetry breaking, the genuine BIC transforms into the high-$Q$ quasi-BIC resonance with tight field confinement for record high THG efficiency of $3.9\times10^{-4}$ W$^{-2}$ and FHG efficiency of $4.8\times10^{-10}$ W$^{-4}$ using a moderate pump intensity of 1 GW/cm$^{2}$. Moreover, the quasi-BIC and the resonantly enhanced harmonics exhibit polarization-insensitive characteristics due to the special $C_{4}$ arrangement of meta-atoms. Our results suggest the way for smart design of efficient and robust nonlinear nanophotonic devices.
分类： 光学 >> 量子光学 提交时间： 2023-02-19
摘要： The research of two-dimensional (2D) materials with atomic-scale thicknesses and unique optical properties has become a frontier in photonics and electronics. Borophene, a newly reported 2D material provides a novel building block for nanoscale materials and devices. We present a simple borophene-based absorption structure to boost the light-borophene interaction via critical coupling in the visible wavelengths. The proposed structure consists of borophene monolayer deposited on a photonic crystal slab backed with a metallic mirror. The numerical simulations and theoretical analysis show that the light absorption of the structure can be remarkably enhanced as high as 99.80% via critical coupling mechanism with guided resonance, and the polarization-dependent absorption behaviors are demonstrated due to the strong anisotropy of borophene. We also examine the tunability of the absorption behaviors by adjusting carrier density and lifetime of borophene, air hole radius in the slab, the incident angle and polarization angle. The proposed absorption structure provides novel access to the flexible and effective manipulation of light-borophene interactions in the visible, and shows a good prospect for the future borophene-based electronic and photonic devices.
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